1. Field of the Invention
This application claims the benefit of priority to Korean Patent Application No. 10-2015-0032669, filed on Mar. 9, 2015, the entire disclosure of which is incorporated herein by reference.
The present invention relates to a method for producing carbon nanostructures in a continuous process by using some of the as-produced carbon nanostructures as fluidic materials, carbon nanostructures produced by the method, and a composite including the carbon nanostructures.
2. Description of the Related Art
Fluidized bed reactors are reactor devices that can be used to carry out a variety of multiphase chemical reactions. In such a fluidized bed reactor, a gas or liquid as a fluid reacts with a particulate solid material. The solid material is typically a catalyst having a small spherical shape, the fluid flows at a velocity sufficient to cause the solid material to float, and as a result, the solid material behaves similarly to the fluid.
Generally, carbon nanostructures (CNSs) refer to nano-sized carbon structures having various shapes, such as nanotubes, nanofibers, fullerenes, nanocones, nanohorns, and nanorods. Carbon nanostructures can be widely utilized in a variety of technological applications because they exhibit excellent characteristics.
Carbon nanotubes (CNTs) as representative carbon nanostructures are tubular materials in which adjacent carbon atoms are bonded together in a hexagonal honeycomb structure and the resulting carbon sheets are rolled into cylinders. Carbon nanotubes exhibit metallic or semiconducting properties depending on their structure, i.e. the orientation of hexagons in the tubes. Due to these characteristics, carbon nanotubes can find a wide range of applications in diverse technological fields. For example, carbon nanotubes are applicable to secondary batteries, fuel cells, electrodes of electrochemical storage devices (e.g., supercapacitors), electromagnetic wave shields, field emission displays, and gas sensors.
Carbon nanotubes can be produced by techniques, such as arc discharge, laser ablation, and chemical vapor deposition. According to chemical vapor deposition, carbon nanostructures are typically formed by dispersing and reacting metal catalyst particles and a gaseous hydrocarbon raw material in a fluidized bed reactor at a high temperature. That is, the metal catalyst reacts with the gaseous raw material while floating in the gaseous raw material in the fluidized bed reactor and carbon nanostructures continue to grow during the reaction.
Methods for producing carbon nanostructures using a fluidized bed reactor are disclosed, for example, in Korean Patent Publication Nos. 10-2009-0073346 and 10-2009-0013503. The fluidized bed reactor uses a distribution plate that permits a gas to be uniformly distributed in the reactor but prevents a powder, such as a catalyst, from passing downwardly therethrough. A perforated plate, a bubble cap, a sieve or a nozzle is generally used as the distribution plate.
In the fluidized bed reactor, the gas flows upwardly through the distribution plate to allow a particle bed on the distribution plate to float in a fluidized state. However, the upward gas flow does not ensure sufficient mixing of the powder with the gas or causes the particles to stay only for a short time in the reactor. In this case, carbon nanostructures tend to aggregate and settle down on the upper surface of the distribution plate due to their strong van der Waals attractive force, the catalyst continues to accumulate on the aggregates, and new carbon nanostructures grow on the catalyst. Thus, the carbon nanostructures increase gradually in size, impeding the fluidity of the gas and the catalyst in the reactor. As a result, satisfactory growth of the gas into carbon nanostructures cannot be expected, causing long operating time or poor product yield. Further, unreacted catalyst is deposited on the distribution plate or clogs the pores of the distribution plate. This obstructs uniform supply of the reactant gas and creates a pressure drop, making it difficult to stably operate the fluidized bed.
According to a batch type method for producing carbon nanostructures, after collection of carbon nanostructures from a reactor, the reactor is cooled and a new catalyst is added thereto. The subsequent heating of the reactor is very time- and cost-consuming and the contact time of a reaction raw material with the catalyst is short. Further, the absence of a fluidic material makes it difficult for the reaction to proceed.